<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>重复请求</title>
</head>

<body>
    <button>点击发送请求</button>
    <script>
        const btns = document.querySelectorAll('button');
        let xhr = null;
        // 标识变量
        let isSending = false; //是否正在发送ajax请求
        btns[0].onclick = function() {
            // 判断表示变量
            if (isSending) xhr.abort(); //如果正在发送则取消该请求，创建一个新的
            xhr = new XMLHttpRequest();

            //修改标识变量的值
            isSending = true;
            xhr.open('GET', 'http://127.0.0.1:8000/delay');
            xhr.send();
            xhr.onreadystatechange = function() {
                //修改标识变量
                if (xhr.readyState === 4) {
                    isSending = false;
                }
            }
        }
    </script>
</body>

</html>